Credits to Val, thank you for your help.
The attached PDF will help you set up your Linksys PAP2T device from scratch. It is a known working configuration so if you are having troubles with your Linksys PAP2T reset back to defaults and follow this guide.
Edit:
1) Navigate to Voice-->SIP-->SIP Timer Values (sec)
a. Reg Retry Intvl should be 120 seconds
https://support.freephoneline.ca/hc/en- ... redentials
b.Mango suggests the SIP T1 default setting is too aggressive, and to help resolve potential registration issues, T1 should be set to 1.
Navigate to Voice-->SIP-->SIP Timer Values-->SIP T1
Change SIP T1 to 1
2) In your PAP2T, Navigate to the SIP tab-->NAT Support Parameters, and make sure that the following settings are enabled:
(Thanks to Mango)
a)Handle VIA received-->yes
b)Handle VIA rport-->yes
c)Substitute VIA Addr-->yes